Not Applicable.
The present invention relates generally to heatsinks, and more particularly, to materials for securing heatsinks to integrated circuits (ICs).
As is known in the art, integrated circuits may be manufactured using a so-called encapsulation process in which an integrated circuit chip or die (i.e. an unpackaged functional element manufactured by subdividing a wafer of semiconductor material) is packaged for mounting and/or protective purposes. During the encapsulation process, a lead frame is disposed in a lead frame holder. A body of the lead frame has projecting therefrom a plurality of pins which form the electrical contact points of a complete integrated circuit. One or more dies may be disposed on a flag portion of the lead frame and wires are bonded to the die(s) and to corresponding ones of the lead frame pins to thus provide a lead frame assembly.
The lead frame assembly is disposed in a mold. An encapsulating material such as plastic, for example, is injected into the mold to enclose the lead frame assembly including the semiconductor die. The resultant package thus corresponds to an encapsulated integrated circuit.
The pins project through the encapsulating housing and thus provide electrical connection points for the integrated circuit. The encapsulating material is proximate to and often physically contacts the semiconductor die.
As is also known, there is a trend to reduce the size of semiconductor devices, integrated circuits and microcircuit modules while having the devices, circuits and modules perform more functions. As a result of the increased functionality, such devices, circuits and modules thus use increasingly more power than heretofore. Such power is typically dissipated as heat generated by the devices, circuits and modules.
This increased heat generation coupled with the need for devices, circuits and modules having relatively small sizes has led to an increase in the amount of heat which must be transferred away from the devices in order to prevent the devices, circuits and modules from becoming destroyed due to exposure to excessive heat. Such devices circuits and modules, are presently limited with respect to the amount of the self-generated heat which they can successfully expel and prevent from building up as they are caused to operate at higher powers.
The proximity of the semiconductor die and the encapsulating housing results in a path between the die and the housing through which heat flows from the semiconductor die to the housing. Heat paths also exist through the bond wires which lead from the semiconductor dies to the pins of the integrated circuit device, however, such heat paths are relatively ineffective due to the relatively small size of the bond wires which are typically provided having a diameter in the range of 0.001 to 0.005 inch. Thus, in most cases it is desirable to extract heat through the surface of the encapsulating housing of the integrated circuit.
The encapsulating material has a mold release characteristic which prevents the encapsulating material from adhering to the mold and thus allows the complete integrated circuit to be separated from the mold in a relatively easy manner and without causing damage to any portion of the integrated circuit. One problem with such encapsulating material, however, is that the mold release characteristic of the encapsulating material prevents other circuit components from adhering to the integrated circuit housing. Thus it is relatively difficult to reliably attach a heatsink to the encapsulating material of the integrated circuit package.
One approach to attaching heatsinks, therefore, has been to mechanically attach the heatsink with a clamp for example. In this approach, a thermal grease or oil is applied to the heatsink, for example, and the heatsink is then placed on the integrated circuit package. A clamp is then used to secure the heatsink to the integrated circuit. One problem with this approach, however, is that the clamps take up space on the printed circuit board to which the clamp is attached. Furthermore, it is relatively time consuming to attach heatsinks to integrated circuits using such clamps.
Moreover, the clamp generates a relatively large compression force between the IC package and the heatsink. The compression force can thus bend and/or distort both the heatsink and the IC package thereby damaging either one or both of the heatsink and IC.
Also, it is relatively difficult to interface such a clamp to an IC package because the clamp attachment points represent areas of very high local stress. If the IC package is plastic, the clamp can locally deform, or even crack the plastic.
Furthermore, the electrical pins of the IC are often located in the regions most desirable for mechanical clamp attachment means. Thus, a relatively complex clamp is often required to properly secure the heatsink to the IC.
Another approach for adhering a heatsink to an IC is to use double sided tape to secure the heatsink to a surface of the integrated circuit. In this approach, a first surface of a strip of double sided tape is placed on the integrated circuit and a second surface of the double sided tape is left exposed. A heatsink is placed on the integrated circuit with at least a portion of the heatsink attached to the second surface of the double sided tape. In this manner the heatsink can be attached to the integrated circuit. One problem with each of these approaches, however, is that due to heating of the integrated circuit, the tape tends to separate from either the integrated circuit package or the heatsink. Thus the heatsink can separate from or fall off the integrated circuit.
In some instances, the environments in which the devices, circuits and modules are used permit complex forced-fluid cooling systems to be employed. Such forced-fluid cooling systems while effective for cooling the devices, circuits and modules are relatively expensive and bulky and require a relatively large amount of space.
Another more economical approach involves the attachment of relatively simple heatsinks having fins provided by metal extrusion or stamping techniques. Such finned heatsinks help to conduct and radiate heat away from the thermally vulnerable regions of the integrated circuit component. For such purposes, it is important that the thermal impedance between a semiconductor or microcircuit device and its associated heatsink structure be kept to a minimum and that it be of uniformity which will prevent build-up of localized hot spots on the device, circuit or module. Such characteristics are not always realized to a satisfactory extent by simply abutting some part of the heat-generating unit with complementary surfaces of its heatsink because, despite appearances, the respective mating surfaces of the heat-generating unit and heatsink will generally have only a relatively small percentage of surface area in actual physical contact.
Such limited contact between a heatsink and an IC component and the attendant poor transmission of heat is due, at least in part, to relatively gross imperfections in the contacting surfaces of the heatsinks and the devices, circuits and modules with which the heatsinks are used. The contact area may be increased somewhat by machining the mating surfaces of the heatsink and the device, circuit or module to relatively precise tolerances. Alternatively, other surface shaping techniques may also be used.
The contact area may also be enlarged by tightly clamping together the heatsink and the device circuit or module. A relatively large clamping force between the heatsink and IC forces irregular surfaces of the heatsink into contact with irregular surfaces of the IC and thus can improve heat transfer characteristics between the heatsink and IC, but the effect is non-linear with a fractional exponent. Thus relatively large increases in force are needed to achieve small improvements in thermal transfer characteristics. As mentioned above, exposure to such large clamp forces can damage the heatsink and/or the IC.
Small surface contact areas can also be attributed, at least in part, to microscopic surface rregularities, which will remain at the interface between a heatsink and a device, circuit or module even if the cooperating parts are formed and finished with great care. When it becomes necessary to electrically insulate one part from another, the heat conduction problems are greatly compounded.
Also among the prior practices which have been employed in efforts to improve the heat flow from semiconductor or like devices to their heatsinks is that of spreading amorphous oil or grease, such as silicon, between the joined surfaces. The messy character of such a filler, as well as the use of insulating mica and varnish insulating layers, are referred to in U.S. Pat. No. 3,29,757 . Thermal grease and/or powdered metal, contained by a film, has likewise been proposed to augment heat transfer, in U.S. Pat. No. 4,092,697 , although the fabrication and handling of such small xe2x80x9cpillowsxe2x80x9d obviously involves special problems also. Cooling fins have been secured by means of epoxy cement loaded with powdered metal (U.S. Pat. No. 3,261,396), and large-area epoxy films have been said to insulate while yet transferring large quantities of heat (U.S. Pat. No. 3,611,046). In U. S. Pat. No. Re. 25,184 , electrically non-conductive plastic coating material is filled with molybdenum disulfide to promote heat conduction, and, where electrical insulation is not essential, a dimpled malleable metal wafer has been interposed to increase transfer of heat (U.S. Pat. No. 4,151,547).
It would, therefore, be desirable to provide a reliable, relatively low-cost technique for removing heat from an integrated circuit. It would also be desirable to provide a technique for reliably mounting a heatsink to an integrated circuit.
In accordance with the present invention, a heatsink assembly includes a heatsink having first and second opposing surfaces with a thermally conductive matrix material disposed over at least a portion of the first surface thereof. With this particular arrangement, a heatsink assembly which may be securely and reliably coupled to a housing of an integrated circuit is provided. The conductive matrix material may be provided having a continuous electrically and thermally conductive scaffold matrix shape which provides a thermal path through an adhesive. The adhesive allows the thermally conductive matrix material to be bonded to the heatsink to thus provide a heatsink assembly which can be adhered to an integrated circuit housing. In a preferred embodiment the heatsink is provided from a Aluminum Silicon Carbide material and a first surface of the heatsink has a plurality of pin or fin shaped structures projecting therefrom. A second surface of the heatsink is provided having a shape which is complementary to that portion of the heat generating device to which the heatsink is to be adhered. For example if the heat generating device corresponds to an integrated circuit component, then the second surface of the heatsink is provided having a surface topology which is complementary to that portion of the integrated circuit housing to which the heatsink is to be adhered. Furthermore, the heatsink and thermally conductive matrix material may be provided having a shape which matches the shape of the portion of the integrated circuit housing to which the heatsink is to be adhered. In a preferred embodiment the thermally conductive matrix material is provided as a type known as W. L. Gore, Inc. and identified as Gore-Bond MG or that shown in FIGS. 9-13 . Typically, it is desired to provide the thermally conductive matrix having a thickness which is as thin as possible to thereby minimize the temperature difference between the heatsink and the silicon die of the integrated circuit component.
In a further embodiment of the invention, a heatsink assembly includes a folded fin member bonded by a thermally conductive matrix material to a plate. In a preferred embodiment, the folded fin member and plate are formed from aluminum so that both the folded fin member and the plate have the same thermal expansion characteristics to reduce stress between these components. This particular arrangement advantageously allows a relatively thin folded fin member to be in thermal communication with the plate to provide a heatsink assembly having a low pressure drop with respect to air flow through the assembly. This heatsink assembly may bonded to an integrated circuit with the same or different thermally conductive material.
In accordance with a further aspect of the present invention, a method for adhering a heatsink to an integrated circuit includes the steps of applying a thermally conductive matrix material to a first one of a first surface of a heatsink or a first surface of an integrated circuit housing and mating the surface having the matrix material disposed thereon to a second one of the first surface of the heatsink and the first surface of the integrated circuit housing. It is desirable to ensure flatness of the first surface of the heatsink, for example by lapping, and to clean the surfaces to be mated to remove grease, oil, or gross contaminants to the maximum extent possible. Cleansing of the surface of the IC may be accomplished with denatured alcohol or acetone, for example. The surface of the heatsink may be cleaned with a mild caustic etch and warm water rinse. It should be noted, however, that other cleaning techniques may also be used and that the particular cleaning materials and techniques should be selected in accordance with a variety of factors including but not limited to the particular materials from which the mating surfaces of the heatsink and IC are manufactured. With this particular arrangement a method for placing a heatsink on a integrated circuit devices is provided. The thermally conductive matrix material may then be exposed to heat at a predetermined temperature for a predetermined amount of time to thus cure the thermally conductive matrix material thereby securely adhering the heatsink to the integrated circuit housing. However, the thermally conductive material can be cured after bonding to the integrated circuit by heat generated by the integrated circuit. The heatsink-integrated circuit assembly may then be disposed on a printed circuit board. The integrated circuit may be soldered to the printed circuit board via any conventional soldering technique including vapor phase soldering techniques. If vapor phase soldering techniques are used, the thermally conductive matrix material on the heatsink and integrated circuit may be cured simultaneously with the soldering of the integrated circuit to the printed circuit board. The thermally conductive matrix material may be cured, for example, by exposure to a temperature of 225 degrees centigrade for a time period typically of about five minutes.
In accordance with a further aspect of the present invention, a printed circuit board assembly includes a heatsink having first and second opposing surfaces with a thermally conductive matrix material disposed over a first surface thereof. The first surface is then disposed over a first surface of an integrated circuit. A second surface of the integrated circuit is disposed over a first surface of a printed circuit board and the integrated circuit is coupled to the printed circuit board. With this particular arrangement, a printed circuit board assembly having a thermally reliable integrated circuit is provided. By placing the thermally conductive matrix material on the heatsink to form a heatsink assembly and placing the heatsink assembly on the integrated circuit, a convenient and effective technique for mounting electronic devices on printed circuit boards is provided. In particular, the present invention effects economies and efficiency in the manufacture and assembly of heatsink-integrated circuit combinations and provides highly reliable and effective high-conductivity thermal couplings.
In another aspect of the invention, a heatsink interface material for securing a heatsink to an integrated circuit is provided. The interface material includes a core material having opposing first and second surfaces, a first region adjacent the first surface, a second region adjacent the second surface, and an intermediate region between the first and second regions. In an exemplary embodiment, an adhesive is disposed on the first and second surfaces and in the first and second regions while the intermediate region is substantially free of the adhesive. Heat and/or pressure can be used to facilitate penetration of the adhesive into the core material. In one embodiment, copper and then nickel can be deposited, such as by electroless deposition, on the first and second regions of the core.